As taught by U.S. Pat. No. 3,753,794, a ductile foil is produced by conventional techniques by reducing or eliminating elements present in the superalloy composition which make the alloy brittle and unworkable. The ductile superalloy foil which differs in composition from the basic superalloy composition is then boronised and the boron is present as high melting point borides which are the reaction product of boron with the substrate elements.
The disadvantages of the present technique is that the superalloy foil composition differs considerably from the parent metal and therefore, less is the potential for homogeneity at the completed joint as described in U.S. Pat. No. 3,753,794. Also the boronizing process enables the boron to react with the refractory and reactive elements present in the superalloy foil, thereby forming, extremely stable, high melting borides which reduce the rate of diffusion of boron away from the joint during the brazing cycle. U.S. Pat. No. 4,038,041 describes a laminated foil wherein the brazing alloy is a composite interlayer of more than one foil, with amounts and composition of each individual foil being such that the overall composition of the composite is equivalent to the parent superalloy composition. Although this invention provides a complete joint which is of the same composition as the parent superalloy, it has the disadvantage of fabricating the brazing alloy interlayer in multiple foils.
U.S. Pat. No. 3,856,513 discloses a wire product where alloys are represented by the formula TiXj wherein T is a transition metal and X is Al, Sb, Be, B, Ge, C, In, P, Si or Sn. The transition metals include metals from Groups IB, IIIB, VB, VIB, VIIB and VIIIB of the periodic table. The patent also teaches that the alloys contain at least 50 percent amorphous phase. As is apparent from that description, 280 binary alloys are disclosed and an infinite number of ternary, quaternary, etc. alloys. Specific examples are given to a Pd.sub.77.5 Cu.sub.6 Si.sub.16.5 alloy and a Ni.sub.40 Pd.sub.40 P.sub.20 alloy. The patent also discloses ternary alloys of the formula M.sub.a Y.sub.b Z.sub.c in sheet, ribbon and powder form wherein M is Fe, Ni, Cr, Co or V, Y is P, C or B and Z is Al, Si, Sb, Ge In or Be.
The present invention is a thin ductile foil of the same composition as the parent superalloy and therefore having a greater potential for homogeneity at the completed joint. The boride coating on the superalloy foil allows for faster diffusion of the boron into the parent metal since the borides of nickel, iron and cobalt are less stable than the borides of refractory and reactive elements. Since the present invention overcomes the disadvantages of the prior art since it has the same metallurgical composition as the parent superalloy it is believed that this invention is an advancement in the art.